The announced retirement this week of two endangered Southern California congressmen, Republicans Darrell Issa and Ed Royce, could signal something larger. If a Democratic wave is building, the swell may be gathering along the Pacific Coast.

California always stood at the center of this year’s fight for control of the House. Democrats, in the minority for most of the decade, need 24 seats to seize control in November. More than half a dozen of their top targets are in California, including the seats held by Issa and Royce.

Significantly, their districts — filled with well-educated suburbanites, social moderates, aspiring immigrants and their millennial offspring — are the very embodiment of the year’s election battleground; not just in California, but in Arizona, Minnesota, New Jersey, Pennsylvania, Texas and Washington state.

Some may be put off by President Trump’s strident language on immigration and his pugilistic tweeting. Others stand to lose under the Republican tax bill, which limits deductions for mortgage interest and state and local taxes.

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Issa, who barely won reelection 14 months ago, was perhaps the most vulnerable Republican congressman in the country, so his departure doesn’t much change the calculation for Democrats, who all but took his ouster for granted.

“I am forever grateful to the people of San Diego, Orange and Riverside counties for their support and affording me the honor of serving them all these years,” the Vista lawmaker, first elected in 2000, said in Wednesday morning’s surprise announcement. “Representing you has been the privilege of a lifetime.”

Royce’s seat is different. The Fullerton Republican was, by most measures, at least decently positioned for reelection. He won his 13th term in 2016 by 15 percentage points, was sitting on $3.5 million in cash and enjoyed strong support in Orange County’s large Asian American community, an asset that other Republicans may be hard-pressed to match.

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Overnight, a race with a seeming GOP tilt now becomes the Democrats’ to lose, enhancing their prospects for November.

Other threatened California Republicans include Jeff Denham of Turlock, Steve Knight of Palmdale, Dana Rohrabacher of Costa Mesa, Mimi Walters of Irvine and David Valadao of Hanford. All save Issa and Rohrabacher voted in favor of the tax bill, which is highly unpopular, according to polls.

But politics is never just about mathematics, as in the number of seats needed to take control.

There is a psychological aspect to the departure of Issa and Royce, and that may prove even more consequential

“It’s sort of like a distress signal,” said Amy Walter, who handicaps elections nationwide for the nonpartisan Cook Political Report. “But instead of signaling help, it suggests a really potentially bad year is on the way.”

How bad it is, of course, is unknowable until the last polling places close on Nov. 6. And here the requisite, post-Trump caveat is necessary: As 2016 showed, any political precedents must be viewed with no small amount of caution.

Still, the 2018 midterm election always figured to be a difficult one for the GOP. Almost without exception, the party controlling the White House loses seats at the halfway point of a president’s term, as the discontented look to state their grievance. That is the case with even the most popular incumbent, and Trump has set modern polling records for voter disapproval.

The question is whether a dismal Republican year turns disastrous, and the danger signs for the president’s party are abundant.

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It’s sort of like a distress signal. But instead of signaling help, it suggests a really potentially bad year is on the way.

Political analyst Amy Walter

The most reliable gauge of a political wave has been the generic ballot, where, simply stated, voters are asked whether they would prefer a Democratic or Republican-run Congress. The Democratic lead is currently substantial, about where it stood in 2006 when a 30-seat gain lifted the party to control of the House.

Further warning rests in the roughly 70 special elections held in 2017, for Congress and also state legislatures and other elected positions throughout the country. Across the board, Democrats did better than previous voting patterns would have suggested, an evident sign of voter engagement lacking on the Republican side.

Perhaps most shocking was last month’s special election in deeply conservative Alabama, where Democrat Doug Jones won his party a Senate seat for the first time in a quarter of a century. (Admittedly, he triumphed over a deeply flawed Republican candidate, Roy Moore.)

“Members of Congress are strategic politicians, and many [Republicans] are making strategic decisions about their lives,” said Stu Rothenberg, another longtime election handicapper. “Do they want to spend the next year raising money, fighting a difficult campaign and at, the end of the day, lose? Or do they want to go out on their own terms and get on with their lives?”

The higher the number of retirements, the greater the advantage for Democrats, as history shows it much easier to win an open seat than one held by an incumbent. With Issa’s exit, the ranks of departing GOP lawmakers rose to 31 nationwide — though some are retiring from the House to pursue other elected offices.

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The last time there was a comparable number of retirements in the majority party was 1994, when 28 Democrats stepped aside. That also happens to be the year the Republicans took control of the House, ending 40 years in the minority.

With their perch overlooking the ocean, Royce and Issa may have seen what was coming. Standing aside was apparently the preferred option to facing a wave and the risk of drowning.

Times staff writer David Lauter in Washington contributed to this article.

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